LIGHT-INDUCED DMA BASE EXCITATION, DEACTIVATION AND OXIDATION DMA bases are the only nucleic acid components that can be electronically excited by solar UV irradiation. Reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide (O2") and hydroxyl radicals (OH) produced by excited bases are cytotoxic and have been implicated in the etiology of a wide array of human diseases. Surprisingly, the detailed mechanisms governing the base excitation and ROS formation are far from being fully understood, which is partly due to the fact that low quantum efficiencies of fluorescence and phosphorescence from DNA bases make the measurements difficult. Quantitative determination of 1O2 and O2~ production from excited bases will be able to surmount this obstacle. Our hypothesis is based on the ideas that the production of ROS is directly related to the excitation and deactivation pathways of DNA bases, which may be controlled by excitation light (energy and intensity) and microenvironments (pH, antioxidants, substituents, etc.). By quantitatively determining the production of 1O2 and Oz~ from excited bases, identifying photooxidation products, testing DNA damage and oxidative stress, this proposal aims to systematically investigate the excitation and deactivation mechanisms of DNA bases and to clarify the key factors controlling the formation of ROS. Specifically, we seek to answer the following questions: How efficiently do the bases, nucleosides and nucleotides produce 1O2 and O2~ via type II and I photosensitization processes (including one- and two-photon absorption mechanisms), respectively? What may be the physical and chemical quenching rate constants of 1O2 by selected bases, nucleosides and nucleotides? What are the effects of excitation energy and intensity on the quantum yields of 1O2 and O2" production? How will the microenvironments (e.g., pH, antioxidants, substituent, solvents, etc.) affect the production of ROS? What may be the targeting sites and oxidative stress of the nucleoside/nucleotide photosensitized 1O2 and O2"? The instruments employed in this research include (1) time-resolved Nd:YAG laser equipped with germanium 1O2 detector, (2) steady-state photolysis setup with wavelengths tunable from 200 to 700 nm, (3) pico-second laser flash photolysis and (4) other analytical techniques, such as NMR, GC/MS, HPLC/MS, fluorescence, UV/Vis, EPR, etc. This project deals with the reactions of light-induced photooxidation in biomedical relevant system. Oxidative DNA damage by UV light is considered to be of serious personal and public health concern. The accomplishment of this project will provide fundamental knowledge for better understanding the mechanism of DNA damage, especially via light-induced self-photooxidation mechanisms.